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Brake fluid that has aged past usefulness — and the reason it has a replacement interval when it never leaks and never gets visibly dirty.

Brake fluid is hygroscopic by design: it absorbs water from the air through the hoses and the reservoir cap, at a few per cent a year. That is deliberate rather than a flaw — dissolved water cannot pool at the low points and rust the system from inside. The cost is that water boils where brake fluid does not, and vapour compresses where liquid does not. Hard use heats the fluid at the calipers, the water in it turns to steam, and the pedal goes long or soft exactly when the brakes are working hardest — a long descent, or repeated heavy stops.

The anti-lock and stability systems feel it before the driver does, which is how a fluid problem becomes a stored code: their hydraulic unit commands pressures it cannot achieve through compressible fluid, and reports a performance fault. The second path is corrosion. Water in the fluid corrodes the system from within, and the debris that follows is exactly the size to jam the small passages and valve seats inside a modulator. A modulator that will not release pressure cleanly very often has old fluid behind it.

Age is the measure, not appearance. Fluid can look perfectly acceptable and be well past its useful life; most manufacturers specify replacement every two years regardless of mileage. If nobody can say when it was last changed, that is the finding — and a fluid change is inexpensive, frequently the entire repair for a hydraulic-performance fault that looks like a failed modulator.